Twins Paradox: Symmetry & Acceleration

[granpa]

when o' moves of course he experiences a loss of simultaneity. how does that contradict anything?

i have no idea what you are trying to say but i am sure its wrong. relativity says that any frame can be treated as being at absolute rest so it can't possibly be true that assuming that one frame really is at absolute rest leads to a contradiction with relativity.

you might want to look at this:
https://www.physicsforums.com/showthread.php?t=235661

 

[paw]

relativity says that any frame can be treated as being at absolute rest so it can't possibly be true that assuming that one frame really is at absolute rest leads to a contradiction with relativity.

Any frame can be treated as if it is at rest; sure. Not at absolute rest though. There is no such thing in SR as absolute rest. You should be more careful with your language.

i have no idea what you are trying to say but i am sure its wrong.

If you have no idea what I'm saying how can you possibly conclude that it's wrong. That's mighty conceited of you.

 

[granpa]

i have no idea what you are trying to say but i am sure its wrong because relativity says that any frame can be treated as being at absolute rest so it can't possibly be true that assuming that one frame really is at absolute rest leads to a contradiction with relativity.

there is no significant difference between being at rest and being at absolute rest.

 

[paw]

there is no significant difference between being at rest and being at absolute rest.

There is a huge difference between rest and absolute rest. Absolute rest implies there is a preferred frame of reference against which an observer could determine some universal 'truth' about what time an event happened, how fast they are moving, their length etc. This is simply not true according to SR and all the experiments that support it.

 

[granpa]

could determine some universal 'truth' about what time an event happened, how fast they are moving, their length etc.

not true at all. if a frame of absolute rest exists it does not follow that one can find it. relativity says that all observers see exactly the same thing that they would see if they were at rest so they can't determine whether they are moving or not. but it does not say that there is no frame of absolute rest. it just says that you can't find it. things don't cease to exist just because you can't see them.

i wish i had a nickel for every time i have had to say that.

 

[matheinste]

Hello granpa

Quote

-------there is no significant difference between being at rest and being at absolute rest.----------

This is just about the biggest mistake you can make in relativity.

Matheinste

 

[granpa]

relativity says that any frame can be treated as being at absolute rest. how then does introducing a frame of absolute rest result in a contradiction with relativity.

relativity doesn't say there is no frame of absolute rest it just says that you can't find it.

 

[matheinste]

Hello again

Quote

-------there is no significant difference between being at rest and being at absolute rest.----------

Whatever else you may say about relativity not denying that an undetectable absolute rest frame may exist the above statement quoted is completely against the idea of relativity. It is no use just repeating the same thing over and over again. The statement is wrong.

Matheinste.

 

[paw]

relativity says that any frame can be treated as being at absolute rest.

You need to provide a reliable source to support this quote or stop repeating it.

'There is no universal frame of reference pervading all of space, so there is no such thing as "absolute motion [rest]"' : Beiser, Arthur. Concepts of Modern Physics, p2.

relativity doesn't say there is no frame of absolute rest it just says that you can't find it.

Again, please cite a reliable reference or stop repeating this.

Saying you can't find an absolute frame is the same as saying it doesn't exist. To argue otherwise would be an article of faith not science.

 

[robphy]

relativity says that any frame can be treated as being at absolute rest. how then does introducing a frame of absolute rest result in a contradiction with relativity.

relativity doesn't say there is no frame of absolute rest it just says that you can't find it.

I haven't been following this thread... but let me offer two comments. (I saw an earlier post of mine quoted.. so I decided to pop in and see what is up.)

Issues regarding paradoxes in relativity are more likely about
the distinction between inertial and noninertial observers,
rather than about observers "at rest" and "in motion".

A resolution in the "rest" vs "absolute rest" debate could arise if one had
mathematical formulation of those concepts rather than just a reliance on imprecise words.

 

[paw]

A resolution in the "rest" vs "absolute rest" debate could arise if one had mathematical formulation of those concepts rather than just a reliance on imprecise words.

I agree with you on the above but I submit that most people reading 'absolute rest' would take that to mean there exists some preferred frame. I think you'd agree that there is no preferred frame in SR. It's very misleading to use the term 'absolute'.

 

[robphy]

I agree with you on the above but I submit that most people reading 'absolute rest' would take that to mean there exists some preferred frame. I think you'd agree that there is no preferred frame in SR. It's very misleading to use the term 'absolute'.

So, here is how I use the word "absolute"... i.e., my definition for "absolute".

But first...
if there is a transformation that let's a certain direction (observer worldline)
in a space[time] diagram be the "vertical axis" (the "observer at rest"),
then there is a choice of axes (frame of reference) where that direction is vertical (that observer is at rest).
[Maybe a little redundant and imprecise, but should be clear enough.]

Now.. "absolute".

In a Euclidean rotation, there are no eigenvectors (i.e. directions left unchanged ["invariant"] by the rotation). So, there are no "absolute directions" in the plane.

In a Galilean boost, there are no [timelike] eigenvectors (i.e. [non-infinite] velocities left unchanged by the boost). This says that there are no preferred timelike observers. Note that there are eigenvectors which could correspond to "absolute velocities"...those which are infinite. In addition, the notions of "absolute time", "absolute simultaneity", and "absolute length of an object" can be used in this context.

In a Lorentz boost, there are again no [timelike] eigenvectors (i.e. [non-lightspeed] velocities left unchanged by the boost). This says that there are also no preferred timelike observers. Note, however, that there are eigenvectors which could correspond to "absolute velocities"... those which are lightspeed velocities.

If in the relativity cases, there were a timelike eigenvector (and it could be transformed to vertical), then I would regard that as the "observer at absolute rest".

So, for me, "absolute"-ness about the eigenvectors of the transformation.
It seems that if you want "absolute rest", one way is to only have the "identity transformation"... i.e. practically no transformation at all... like the so-called Aristotelian spacetime. One could also dream up a more complicated transformation law to have a timelike eigenvector... but it would probably be accompanied by some possibly-unappealing properties [including disagreement with experiments].

 

[paw]

So, for me, "absolute"-ness about the eigenvectors of the transformation.
It seems that if you want "absolute rest", one way is to only have the "identity transformation"... i.e. practically no transformation at all... like the so-called Aristotelian spacetime. One could also dream up a more complicated transformation law to have a timelike eigenvector... but it would probably be accompanied by some possibly-unappealing properties [including disagreement with experiments].

If I remember the identity transformation is the trivial one that give you back the original matrix right?

Anyway, you don't seem to be saying anything that disagrees in any substantial way with the quote from Beiser above that there is no 'absolute motion [rest]'. If there were it would qualify as a preferred frame and the way I understand that it would imply a positive result for MM type experiments.

I think that for introducing SR or for casual discussions the statement that there is no preferred frame should be sufficient, no?

 

[robphy]

If I remember the identity transformation is the trivial one that give you back the original matrix right?

Anyway, you don't seem to be saying anything that disagrees in any substantial way with the quote from Beiser above that there is no 'absolute motion [rest]'. If there were it would qualify as a preferred frame and the way I understand that it would imply a positive result for MM type experiments.

I think that for introducing SR or for casual discussions the statement that there is no preferred frame should be sufficient, no?

Yes, the identity transformation gives you back the original matrix.

I don't think what I have said is at odds with any relativity text...
What it does, I hope, is to clarify physical concepts with some specific definitions and appropriate mathematical formulations which hopefully capture the essence of these physical concepts. Then, any debates in terminology can shift to a discussion in the context of the mathematical formulation.

 

[granpa]

Saying you can't find an absolute frame is the same as saying it doesn't exist. To argue otherwise would be an article of faith not science.

so if you can't see it then it doesn't exist and to say otherwise is unscientific? ok. fine. it doesn't exist then.

Whatever else you may say about relativity not denying that an undetectable absolute rest frame may exist the above statement quoted is completely against the idea of relativity.

as i pointed out relativity says that any frame may be considered to be at absolute rest. please explain therefore how considering one frame to be at absolute rest leads to a contradiction with relativity.

I agree with you on the above but I submit that most people reading 'absolute rest' would take that to mean there exists some preferred frame.

preferred for what?

 

[RandallB]

relativity doesn't say there is no frame of absolute rest it just says that you can't find it.

A resolution in the "rest" vs "absolute rest" debate could arise if one had mathematical formulation of those concepts rather than just a reliance on imprecise words.

But there is a “mathematical formulation” for it. The The Science of Astrophysics as we know it could not exist without it.

What does not exist is a theoretical basis for establishing a preferred frame of reference to identify something to use as a frame of “absolute rest” when restricted to Special Relativity, but the science of Astrophysics is not limited to SR it uses GR and real world observations.

The idea that there is no “Preferred Frame” to use as a reference for “at rest” only applies when limited to SR alone.

 

[paw]

But there is a “mathematical formulation” for it. The The Science of Astrophysics as we know it could not exist without it.

What does not exist is a theoretical basis for establishing a preferred frame of reference to identify something to use as a frame of “absolute rest” when restricted to Special Relativity, but the science of Astrophysics is not limited to SR it uses GR and real world observations.

The idea that there is no “Preferred Frame” to use as a reference for “at rest” only applies when limited to SR alone.

I'm not disagreeing with you but I thought the discussion was about Special Relativity. In that context I believe I'm correct in my assertion that there is no preferred, or absolute, frame of reference.

I have much less experience with GR although I must say I have never seen a claim that a preferred or absolute frame should exist in GR either. Please correct me if I'm wrong.

 

[paw]

Whatever else you may say about relativity not denying that an undetectable absolute rest frame may exist the above statement quoted is completely against the idea of relativity.

On the contrary. The lack of a frame at absolute rest is fundamental to SR. If such a frame existed and two non-local events in that frame were simultaneous then all other frames would have to also agree that they were simultaneous. But we already know that any frame in motion relative to the absolute frame will not agree they are simultaneous.

as i pointed out relativity says that any frame may be considered to be at absolute rest. please explain therefore how considering one frame to be at absolute rest leads to a contradiction with relativity.

SR says any frame may be considered to be at rest not at absolute rest. There is a difference. If you are confusing rest with absolute rest then Ok but if you are insisting on absolute rest then I ask again; please provide a citation to support your claim.

preferred for what?

In this context a preferred frame is one with the property of absolute rest. That is, one in which 'motion' is absolute. The idea of an ether (aether if you prefer) is a preferred frame. A frame of reference can be defined in which the ether is at rest.

As for your stated notion that an 'absolute frame exists but it can't be detected' I won't go as far as to say that's nonsense but I will say as a working hypothesis it's useless. You might as well argue fairies exist but you can't detect them. If your 'undetectable' frame has no effect on physical reality then you might as well abandon the idea. If it does affect reality then it is detectable in principle.

 

[matheinste]

Hello paw.

Please look at post #43 and you will see that your first quote is what i said in response to granpa saying that there is no significant difference between a rest frame and an absolute rest frame. My words agree with your view.

Matheinste.

 

[paw]

Hello paw.

Please look at post #43 and you will see that your first quote is what i said in response to granpa saying that there is no significant difference between a rest frame and an absolute rest frame. My words agree with your view.

Matheinste.

I already saw it and agree. I was just having one more try to get the point across to granpa. I'm un-likely to succeed I know but what the heck...

 

[granpa]

i refer you to the last paragraph of the op of this thread:

https://www.physicsforums.com/showthread.php?t=236978

that is my final word on aether. agree or disagree as you like.

 

[paw]

i refer you to the last paragraph of the op of this thread:

https://www.physicsforums.com/showthread.php?t=236978

that is my final word on aether. agree or disagree as you like.

Quoting yourself is hardly authoritative support for your own argument. However, after reading your lengthy and convoluted post I see you aren't disagreeing with SR, rather, you are adding an unsupported, unnecessary and confusing layer of complexity to it. I think it would be better to drop the idea of a preferred frame entirely instead of saying one may exist but can't be detected. It's just simpler all around.

 

[granpa]

where do you read anything about a preferred frame??

i have dropped the whole issue of an aether altogether. that post was supposed to be plain and simple special relativity. did you not read the part about 'there is no absolute velocity'? it sounds like you just want to argue to me.

and i wasnt quoting myself. i was answering your question. why are you being so argumentative? chill out.

 

[paw]

where do you read anything about a preferred frame??

i have dropped the whole issue of an aether altogether. that post was supposed to be plain and simple special relativity. did you not read the part about 'there is no absolute velocity'? it sounds like you just want to argue to me.

and i wasnt quoting myself. i was answering your question. why are you being so argumentative? chill out.

Geez, you didn't read my post too carefully. I said your link showed you weren't disagreeing with SR. We agree that there 'is no absolute velocity'. I'm glad that's finally clear. It was the point I was making all along.

 

[granpa]

there is nothing in that thread about a preferred frame. and every word of it is entirely supported mathematically.

 

[mysearch]

Question related to post #26/Granpa

This response references post #26 so apologises for being so out-of-sync with the current exchanges. However, an aspect of this thread was raised in another thread entitled `Gravitational Redshift`, which touched on a triplet extension to the twin paradox – outlined below.

I was particular interested in the link given in #26.
http://www.sysmatrix.net/~kavs/kjs/addend4.html

Initially I thought this link was trying to explain how 2 frames of reference could both justify how time was running slower in the other frame. Having now having had a chance to take a closer look at the detail, I don’t believe the example supports such a conclusion. While it may resolve the time difference between the 2 frames, it is clear at the start and end of the journey that only 1 frame was moving with respect to the other and it was this frame that underwent time dilation.

Note: There is no inference of an absolute frame of reference being made in this statement, simply that because there is an unambiguous start/stop point, where the frames recombined, there is no ambiguity of relative velocity or time dilation.

The triplet variant, mentioned above, is just an extension of the twin paradox. However, while 1 triplet stays on Earth, the other 2 take identical journeys at the same relative speed (and acceleration), as each other with respect to the stay-at-home triplet, but always in the opposite direction, i.e.

Triplet-1: A
Triplet-2: A-B-A-C-A
Triplet-3: A-C-A-B-A

Calculations, based on special relativity, seem to suggest that triplet 2 & 3 both measure the same elapsed time, which is less than triplet-1, at the end of the journey. However, there is a point in the journey above, when triplets 2 & 3 pass each other, as well as triplet-1, at point (A), where all have a relativistic velocities with respect to each other. This suggests that triplets 2 and 3 must experience some relativistic time dilation, with respect to each other, while the overall suggestion is that triplets 2 and 3 mark the same time throughout the journey with respect to triplet-1.

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_vase.html#doppler

The link above outlines both the Doppler Shift and GR explanations of the twin paradox based on a relative velocity of 0.866c giving $$\gamma=2$$. As such, it is the ‘stationary’ twin that transmits twice as many light pulses and the ‘moving’ twin. It can be seen that the ‘moving’ twin always receives twice as many pulses than it transmits over the entire journey due to the effects of time dilation. While the change in relative velocity makes the arrival rate complex, it is not impossible to calculate exactly when the pulses will arrive, assuming that the time dilation is a constant ongoing effect.

So finally, applying this same analysis to the triplet example, it seems to suggest that time for triplets 2 and 3 runs at the same rate throughout the journey, which is only time dilated with respect to triplet-1. If so, it suggests that no ‘physical or meaningful’ time dilation takes place as triplets 2 and 3 pass each other at 0.866c+0.866c=0.99c. Therefore, would be interested in any other interpretations.

 

[RandallB]

I'm not disagreeing with you but I thought the discussion was about Special Relativity. In that context I believe I'm correct in my assertion that there is no preferred, or absolute, frame of reference.

I have much less experience with GR although I must say I have never seen a claim that a preferred or absolute frame should exist in GR either. Please correct me if I'm wrong.

Actually I thought this started as NOT about SR, but about the twin paradox and how or if it relates to GR in an effort to understand the Twins at a level somewhere beyond SR alone.

All I added was that the “assertion that there is no preferred, or absolute, frame of reference” only applies to the on paper interpretation of a SR world. And cannot be automatically extended as true for a complete description of reality, as Astrophysicists must observe reality.
Since 1908 or so it has been known that SR was not capable of a complete description of reality; that is why of more advanced relativistic theories continued to develop beyond SR.

At a minimum when you assert “no preferred frame” it should be better stated as “no preferred frame within the limits of SR”. Otherwise you are likely to miss the simple fact (which I think you are missing) that Astrophysics does require the use of a particular form of preferred frame of reference. Do not think that the idea of a preferred frame goes un-used in physics.

 

[paw]

Actually I thought this started as NOT about SR, but about the twin paradox and how or if it relates to GR in an effort to understand the Twins at a level somewhere beyond SR alone.

Well you may be right, although I don't see any explicit reference to GR in the OP. However, all the comments I've made were in reference to an absolute frame in SR. I did try to make that clear a number of times but I guess it wasn't clear enough.

All I added was that the “assertion that there is no preferred, or absolute, frame of reference” only applies to the on paper interpretation of a SR world. And cannot be automatically extended as true for a complete description of reality, as Astrophysicists must observe reality.

Sure, I understand that.

Since 1908 or so it has been known that SR was not capable of a complete description of reality; that is why of more advanced relativistic theories continued to develop beyond SR.

I agree.

At a minimum when you assert “no preferred frame” it should be better stated as “no preferred frame within the limits of SR”.

I tried but I'll keep in mind in the future to be even more explicit.

Otherwise you are likely to miss the simple fact (which I think you are missing) that Astrophysics does require the use of a particular form of preferred frame of reference. Do not think that the idea of a preferred frame goes un-used in physics.

I am aware that astrophysisists use preferred frames to solve certain problems. I believe this is done for convenience in most cases, although sometimes I think it's used to explore new ideas as well. I don't think it's ever been stated that preferred frames in this context actually represent physical reality though. For example, I haven't read anything coming from the astrophysical community claiming there IS some form of absolute motion; at least in the sense that two simultaneous events in the absolute frame would be simutaneous in all other inertial frames. Please correct me if I'm wrong in this?

 

[RandallB]

Preferred Frames not in SR are used in Astrophysics

I am aware that astrophysisists use preferred frames to solve certain problems. I believe this is done for convenience in most cases, although sometimes I think it's used to explore new ideas as well. I don't think it's ever been stated that preferred frames in this context actually represent physical reality though. For example, I haven't read anything coming from the astrophysical community claiming there IS some form of absolute motion; at least in the sense that two simultaneous events in the absolute frame would be simutaneous in all other inertial frames. Please correct me if I'm wrong in this?

So as not to hijack this thread with the issue of preferred reference frames I’ve opened a separate thread to respond. https://www.physicsforums.com/showthread.php?t=237570"

https://www.physicsforums.com/showthread.php?t=237570